This invention relates to radiological well logging methods for investigating the characteristics of subsurface earth formations traversed by a borehole and, more particularly, relates to improved neutron-gamma ray logging methods for determining the chlorine content of earth formations.
It is well known that oil and gas are more likely to be found in commercially recoverable quantities from relatively porous earth formations than from more highly consolidated and less permeable earth formations. It is also known that an oil or gas producing formation may be located by passing a neutron source through the borehole and measuring the intensity of secondary gamma ray radiation developing from the neutron irradiation as a function of borehole depth.
In particular the chlorine nucleus, which has a very high thermal neutron capture cross section (more so than that of the nuclei of other rather commonly found elements), is a good indicator of the location of salt water in subsurface earth formations. Thus, salt water filled limestone or sandstone layers will have a greater macroscopic thermal neutron capture cross section than similar formations which are oil saturated. By combining other porosity information, such as that detected by sonic or density logging tools, with the thermal neutron capture cross section data oil can be located. The macroscopic thermal neutron capture cross section has been observed in the past by measuring either chlorine characteristic thermal neutron capture gamma rays or the lifetime or decay constant of the thermal neutron population in the subsurface earth layer being investigated.
The above-mentioned salt water detection techniques have proven to be very useful in the past in locating oil and gas bearing earth formations. However, many spurious indications have been produced by depending solely on thermal neutron capture logging techniques due to the fact that the techniques require the presence of salinity in rather large amounts to be reliable. There has been no commercially available well logging method which could distinguish oil from water in earth formations where the water salinity was low. For example, the above-mentioned chlorine capture (of neutron lifetime) logs require water salinities in excess of about 30,000 to 100,000 parts per million sodium chloride before oil located in the pores of the earth formations can be reliably differentiated from water. The present invention combines inelastic gamma ray scattering measurements with thermal neutron capture gamma ray measurements to overcome some of these difficulties.
It has been proposed in the prior art to make a measurement of at least a portion of the inelastic neutron scattering gamma ray energy spectrum from neutron irradiated earth formations. This has been proposed because carbon and oxygen nuclei in the earth formations surrounding the borehole can engage in appreciable inelastic fast neutron scattering even though the thermal neutron capture cross section of these nuclei is relatively small. This type of fast neutron interaction logging has been limited in the past to some extent because the inelastic neutron scattering cross sections for carbon and oxygen only become appreciable if relatively high energy neutrons are available to provide the interaction. In the past it has been difficult to provide sufficient quantities of highly energetic neutrons to reliably perform this type of log. The development of improved electronically pulsed high energy neutron generators has made possible the measurement of the inelastic fast neutron scattering gamma ray energy spectrum from relatively high energy neutron irradiated earth formations. Attempts have been made to measure the carbon and oxygen inelastic neutron scattering interactions with 14MEV neutrons generated in pulsed neutron generators of the deuterium-tritium reaction type.
A problem which has arisen has been due to the fact that characteristic gamma rays generated in neutron inelastic scattering are themselves scattered by other electrons and nuclei. This generally tends to make the scattered gamma ray lose energy to some extent with each such Compton scattering interaction. Thus, a higher energy gamma ray having a characteristic initial energy resulting from an inelastic scattering event can appear to have totally different lower energy by the time it reaches the detector in the logging sonde due to the multiple Compton scatterings. This type of gamma ray scattering process generally masks or smears characteristic peaks which might ordinarily occur in the inelastic gamma ray energy spectrum.
In order to obtain reasonable count rates at reasonable source to detector spacings it is desirable to repetitively generate the high energy neutron pulses at as high a repitition rate as practicable. However, when earth formations are repetitively irradiated by a pulsed neutron source at a very high repetition rate, some neutrons may still linger from the previous neutron pulse (in a thermalized condition) when the next or subsequent neutron pulse is emitted. Since these neutrons are thermalized, they can cause thermal neutron capture interactions producing thermal neutron capture gamma rays which could, during the subsequent neutron pulse when the inelastic scattering interactions occur, tend to be confused with the inelastic scattering gamma rays sought to be measured. This could occur because the inelastic neutron scattering gamma ray energy spectrum which is sought to be measured must be measured essentially only during the time interval that the neutron generator is turned on. This occurs because the population of high energy neutrons falls off very quickly due to the inelastic neutron scattering phenomena itself being a relatively strong nuclear interaction and because of elastic neutron scattering with the hydrogen present. This problem may be overcome, however, by employing certain background correction techniques such as those disclosed in U.S. Pat. No. 3,780,303 which is assigned to the assignee of the present invention.
The system disclosed in the aforementioned patent utilizes a plurality of energy dependent windows or intervals to observe the relative count rates in selected portions of the inelastic neutron scattering gamma ray energy spectrum. Time dependent gating means isolate gamma rays resulting from the inelastic scattering of neutrons by the earth formations surrounding the well borehole. The timing means also provide a separate time dependent isolation of thermal neutron capture gamma rays just prior to each high energy neutron pulse. Gamma rays observed during this separate interval are primarily the result of neutron capture interactions caused by any thermal neutrons remaining in the vicinity of the gamma ray detector. Of course, some gamma rays due to natural formation radioactivity and some gamma rays due to neutron activation of the formation elements may also be detected in this manner. The separate time dependent isolation of gamma rays just prior to each neutron pulse provides a background estimate of remaining thermal neutrons. Energy ranges or "windows" in the selected portions of the inelastic gamma ray energy spectrum are positioned and their width or energy range chosen so that the characteristic gamma rays resulting from inelastic scattering of fast neutrons (hereafter referred to as inelastic gamma rays) from carbon, oxygen, silicon and calcium may be detected. The carbon and oxygen inelastic gamma rays may be used to provide an estimate of the hydrocarbon content of the earth formations which surround the borehole. The calcium and silicon inelastic gamma rays provide an indication of the lithology of the earth formations (i.e. sand or lime formations). Gamma ray counts occurring in the same carbon, oxygen, silicon and calcium energy windows during the separate background time isolation period and resulting primarily from thermal neutron capture may then be subtracted from the counts due to gamma rays resulting from the inelastic scattering interactions. This background subtraction technique enables the repetition rate of the high energy neutron pulses to be increased while not introducing unwanted counts from capture gamma rays caused by thermal neutrons.
By time isolating a third separate portion and appropriately choosing the location of energy windows in the capture gamma ray energy spectrum, capture gamma rays produced by silicon and calcium can also be measured and developed into a capture calcium (Ca/Si) ratio. This ratio, like the inelastic Ca/Si ratio, can be used to assist in lithology identification. In the capture Ca/Si ratio, however, there is a strong salinity dependence, unlike the situation with the inelastic ratio of the same two elements. This occurs because of the relative similarity of the capture gamma ray spectra of clacium and chlorine. This salinity dependence limits the usefulness of a capture Ca/Si ratio as a lithology indicator, but can be used to differentiate saline from non-saline intervals. To accomplish this, the inelastic Ca/Si and capture Ca/Si ratios are compared (possibly by overlaying log curves) across a section of a well; in zones at anomalous salinity the ratios diverge, whereas in other zones the ratios behave similarly.
The features and advantages of the present invention are pointed out with particularity in the appended claims. The present invention is best understood by taking the following detailed description in conjunction with the appended drawings in which: